Skip to main content

Full text of "The Cambridge Natural History"

See other formats


EARLY   TERTIARY   MAMMALS

be devoid of mammalian remains. This gap, however, has been
filled up by the discovery of mammalian remains in the ^Torth
American Laramic formation, which seems to be clearly of Creta-
ceous age. Furthermore, it is held by some that the Purbeck beds
are more properly to be placed with the Cretaceous, which would
then necessitate the consideration under the present heading of
some of the types already dealt with; and if, as is suggested in
the following section, the lowest so-called Eocene beds are really
referable to the Cretaceous, there is no lack of mammalian remains
in that period. And, moreover, it was in that case the Creta-
ceous period which witnessed the evolution of the existing orders
of Placental mammals. Otherwise the mammalian remains of the
Cretaceous agree with those of the Jurassic. *We find remains
of the Multituberculata in fragments of Plagiaulacidae and
Polyniastodontidae. Ptilodus is a genus which has two pre-
molars; and Meniscoessus is another niultituberculate from the
same Laramic formation. The other detached fragments of
mammals are thought by Osborn to represent both Plaeentals
and Marsupials.
The Mammals of the Tertiary Period.—Unless the lowest
beds of the earliest Tertiary period, the Eocene, such as the
Torrejon of North America, should be in reality referred to the
Cretaceous, there is no evidence that the modern groups of
Mammalia existed before the present epoch of the earth's history.
It is probable, however, that the Eutheria as a group were Meso-
zoic. The fossil jaws that have been considered in the last chapter
may quite probably be primitive Eutherians, or even divisible, as
believed by Professor Osborn, into Marsupials and Insectivores.
In the Tertiary, however, apart from the question as to the
nature of the Puerco and Torrejon formations, and as to certain
South American strata whose fossil contents have been investi-
gated by Professor Ameghino, we find the first traces of mammals
definitely referable to existing orders, or to be distinctly com-
pared with existing orders. Since, however, representatives of
types which have obvious relationships to modern types appear
in considerable profusion in the very earliest strata of the
.Eocene, it seems clear that much remains to be discovered in
beds earlier than these. Confining ourselves, however, to facts
and to comparisons which can be made on more than a few
lower jaws and scattered teeth, which is practically all that we